The field of the invention is semiconductor quantum well lasers.
Semiconductor lasers are the fundamental building block in compact optic and optoelectronic devices. Formed from Group III-V semiconductors, the semiconductor lasers emit laser light in response to electrical stimulation as electrons relax back to lower energy states and emit photons. Two conventional types of semiconductor lasers are buried heterostructure lasers (BH) and ridge waveguide (RW) lasers.
BH lasers are extremely effective at confining carriers. The lateral heterostructure of a BH also creates a large index step, which strongly confines the optical mode of the laser. Another result of the large index step, though, is the support of higher order modes. The higher order modes can give rise to beam instability, a large diffraction angle, and poor fiber coupling efficiency. A physically narrow BH device can defeat propagation of higher order modes, but provides a smaller available gain volume, higher optical power density at its facets, and larger diffraction angle of its emitted beam. Manufacturing narrower BH lasers also poses more difficult manufacture process control problems compared to otherwise similar wider devices. In general, the BH lasers are low threshold but high performance devices.
RW lasers can be made with a comparably smaller index step. The index step of an RW laser is controlled by controlling the depth of the ridge etch. RW lasers are easier to manufacture than BH lasers since the RW lasers require only a single crystal growth step. However, the RW lasers are less efficient than BH lasers. Due to unconfined spreading of carriers, a region outside of the ridge in an RW laser is also pumped leading to gain which is not effectively utilized. As a result, the threshold current of a RW laser can be twice as high as a comparable BH laser.
A semiconductor quantum well laser of the invention utilizes separate lateral confinement of injected carriers and the optical mode. A ridge waveguide is used to confine the optical mode. A buried heterostructure confines injected carriers. A preferred embodiment laser of the invention is a layered semiconductor structure including optical confinement layers. A buried heterojunction within the optical confinement layers is defined in a quantum well layer, and is dimensioned and arranged to confine injected carriers during laser operation. A ridge waveguide outside the optical confinement layers is dimensioned and arranged with respect to the buried heterojunction to confine an optical mode during laser operation. An index step created by the buried heterojunction is substantially removed from the optical mode.